Closed type kneaders which include a pair of kneading rotors supported in a kneading tank so that a kneading material is kneaded under pressure by rotation of the pair of kneading rotors have been generally known, not to mention an example. The known batch type kneaders for kneading a high viscosity kneading material such as rubber, plastic and ceramics in a batch manner include open type kneaders and closed pressure type kneaders. In the closed pressure type kneaders have a problem in that, when a batch of powder compounding agent is loaded into a kneading tank, a gas of a volume which corresponds to that of the powder compounding agent flowing into the kneading tank is ejected while entraining the powder compounding agent, and the gas in the pressurizing cap raising and lowering path is compressed and increased in pressure when the pressurizing cap is lowered, and a dust-containing gas which entrains the powder compounding agent having a volume replaced from the inside of the kneading tank to the upper area of the pressurizing cap as the pressurizing cap is lowered is ejected through a narrow gap between the pressurizing cap and four circumferential inner walls of the raising and lowering path to the outside of the device, and accordingly, dust must be collected by providing a suction hood having a high suction capacity to prevent dust from being scattered to the outside.
The kneader disclosed in Patent Document 1 includes an internal pressure increase prevention device 10 to which a pressure adjusting air bag 28 which is inflatable and deflatable via a chute 34 is connected at a position on a side wall above a kneading tank 16, and is configured to allow a gas in the tank to flow into the air bag when an internal pressure of the kneading tank increases, and to allow the gas in the air bag to be returned into the kneading tank when the internal pressure of the kneading tank decreases. The internal pressure increase prevention device 10 is effective to prevent increase in the internal pressure of the kneading tank when the pressurizing cap pushes the kneading material into the kneading tank and applies pressure to a volume of a gas which corresponds to the amount that the pressurizing cap is lowered, and prevent increase in pressure that causes the powder compounding agent loaded in the kneading tank to be leaked from a shaft seal structure of the kneader or a seal section of a door of a loading section or a output section of the kneading material. In addition to that, the internal pressure increase prevention device 10 also prevents degradation of surrounding environment caused by the powder compounding agent loaded in the kneading tank of the closed pressure type kneader being ejected through a gap around the opening of the kneading tank to the outside.
More specifically, in the kneader disclosed in Patent Document 1, when the gas increased in pressure in the kneading tank as the pressurizing cap is lowered into the kneading tank is ejected through a gap around the pressurizing cap at the opening of the kneading tank, a dust flow which entrains the powder compounding agent is ejected to an area above the pressurizing cap. Since the kneader is equipped with the internal pressure increase prevention device 10 which includes the air bag 28, a mixed phase gas containing the powder compounding agent is stored with the accumulated pressure in the air bag, and then is returned into the kneading tank when an instantaneous pressure drop occurs in the kneading tank due to a raising operation of the pressurizing cap, and accordingly, the powder compounding agent ejected to an area above the pressurizing cap as a dust flow is returned into the kneading material during kneading process.
The kneader can decrease contamination due to dust scattering around the kneader compared to a kneader which does not include the internal pressure increase prevention device, effectively improve a working environment which has been worsened by ejected powder compounding agent such as carbon black and white filler, and also improve the compound quality by recovering the powder compounding agent and returning it to the kneading material. However, the valuable recovery rate of the powder compounding agent remains around 50%, which is not always satisfactory for the following reasons.
That is, in the internal pressure increase prevention device 10 according to Patent Document 1, the air bag body 30 needs a capacity of at least the amount of air which is moved by upward and downward motion of the ram cylinder 24. However, with this capacity, the volume of the gas which flows into the air bag body 30 may exceed the air bag capacity when the material larger than the planned volume is loaded into the kneading tank or when the temperature of the gas in the kneading tank increases. Accordingly, a branch duct 50 which forms a safety flow path for preventing damage is connected to the air bag body 30 so that part of the gas can be exhausted through the branch duct 50 to the external dust collector when the gas which flows into the air bag exceeds the capacity of the air bag.
In the aforementioned configuration of the internal pressure increase prevention device 10, the air bag 28 which is connected to the kneading tank through the chute 34 has a relatively large diameter since it needs a capacity that allows a large amount of gas to be flowed therein. As a result, the gas which flows into the air bag 28 via the chute 34 suddenly decreases in flow speed, and the powder of large particle diameter in the mixed phase flow in the powder compounding agent is precipitated and deposited at an early stage, and the powder of middle particle diameter is suspended and accumulated in the air bag 28 as a mixed gaseous phase, and the powder of fine particles is suctioned through the branch duct 50 to the external dust collector. This may lead to constraints of the recovery rate and the recovery amount depending on the particle diameter and the weight of the powder compounding agent, that is, when the pressurizing cap is lowered at high speed or a large amount of the powder compounding agent is loaded into the kneading tank, a replacement gas which is urged to flow out of the kneading tank is suctioned via the branch duct 50 to the dust collector while entraining high density of fine particles of the powder compounding agent.
Accordingly, regardless of presence of the air bag 28, the gas exhausted through the branch duct 50 to the dust collector contains a large amount of powder compounding agent as dust. Particularly, fine dust cannot be recovered from the exhaust gas exhausted from the branch duct. As a result, although improvement in working environment may be expected, various powder compounding agent ejected into the air bag 28 cannot be efficiently recovered to return to the kneading material. Since almost half of the powder compounding agent which is ejected from the kneading tank is disposed of as dust, there is a need to further improve the valuable recovery rate so as to increase the valuable recovery rate to bring the compound quality close to the design quality.